(1) Field of the Invention
The present invention relates to a so-called "tandem-type" image forming apparatus, and especially relates to an image forming condition adjusting technique used in such tandem-type image forming apparatuses.
(2) Related Art
Images formed by an image forming apparatus are adversely affected by wear and tear on the apparatus and change of surrounding conditions, so that image forming conditions, especially a transfer output, need to be adjusted for image transfer. To adjust the transfer output, not only the amount of toner attracted to a recording sheet or a transport belt (referred to as the "attracted toner" hereinafter), but also the amount of toner reattracted after image transfer (referred to as the "reattracted toner" hereinafter) needs to be considered. Here, the reattraction of toner means a state where toner having attracted to the recording sheet or the transport belt returns to an image holding component, namely, a photosensitive drum.
When only the attracted toner is considered, the transfer output is set so that an adequate amount of toner can be reliably attracted to the recording sheet. However, when the transfer output is set too high, the reattraction of toner occurs. More specifically, when the transfer output is set too high, a potential difference between the photosensitive drum and a transfer material, such as a transport belt, is considerably large. This causes electric discharge by Paschen's law when the recording sheet is separated from the photosensitive drum after image transfer. Due to this electric discharge, the same amount of positive charge and negative charge are produced in a space between the photosensitive drum and the part of the recording sheet to which no toner has been attracted. When image transfer is performed using a positive electric field, for example, the negative charge produced in that space is attracted to the transport belt via the recording sheet which has absorbed moisture in the air. Meanwhile, the positive charge produced in that space neutralizes the negatively charged toner on the recording sheet, or may positively charge the toner. The toner positively charged in this way leaves the recording sheet and is attracted to the negative electric field of the photosensitive drum, thereby causing the reattraction. This reattraction may cause blank spots on the reproduced image, so that it has to be prevented from occurring as much as possible.
To address this problem, a table is stored in a conventional image forming apparatus so that the transfer output is set to keep an appropriate balance between the attraction and reattraction of toner. The table stores transfer outputs having experimentally obtained corresponding to surrounding conditions, such as temperature and humidity inside the copier. Sensors detect the surrounding conditions, and then the transfer output is set using the table in accordance with the detected surrounding conditions.
Moreover, another technique has been suggested, by which an optical sensor detects toner reattracted to the photosensitive drum after image transfer and the transfer output is adjusted so that the reattracted toner is kept to the minimum.
However, by means of the method for adjusting the transfer output using the sensors which detect the surrounding conditions, it is difficult to determined an optimum transfer output since the actual amounts of the attracted toner and reattracted toner are not detected. More specifically, there are still problems caused by unevenness of the charge level of toner on the photosensitive drum due to toner characteristics caused in manufacturing and by instability of detection precision of a temperature sensor and a humidity sensor.
Meanwhile, by means of the method for detecting the reattracted toner on the photosensitive drum using the optical sensor, only the reattracted toner is detected, so that the transfer output based on the attracted toner cannot be set. In addition, if the attracted toner is to be detected, an optical sensor is separately required, thereby increasing cost.
Accordingly, an example where the transfer output is adjusted using detection results of the attracted and reattracted toner has been described. Note that the detection results can also be used for adjusting other image forming conditions, such as an exposure condition.